Methane liquefaction process



SePf- 13, 1966 J. B. MAHER ETAL METHNE LIQUEFCTION PROGESS QN wmooov WM .Loom

Filed Jan. 8, 1964' 'Ummm Oom WW Wl@ www. Si .n.mLOm 6.5L om\ NNW N 5 Vfuzom: MN i .uoOm .1

NNx Ow %N United States Patent O 3,271,965 METHANE LHQUEFACTION PROCESS James B. Maher and James Mair, Chicago, Ill., assignors to Chicago Bridge & Iron Company, Oak Brook, Ill., a corporation of Illinois Filed Jan. 8, 1964, Ser. No. 336,560 9 Claims. (Cl. 62-23) This invention relates to apparatus and methods for storing in liquid form materials which are normally gases at ambient temperatures and pressures. More particularly, this invention is concerned with novel processes and apparatus for liquefying methane, such as in the form of natural gas, and subsequently storing the gas in liquefied form at veryl low temperatures and about atmospheric pressure.

Large amounts of methane, usually in the form of natural gas, are used in heating residences, apartments, otiices and public buildings, as well as in cooking. ln addition, industry employs large amounts of the gas in processing operations of a wide variety.

The demand for gas varies seasonally as well as daily. Larger amounts of gas are needed during the winter season in the northern parts of the United States than during the warmereperiods of the year. The same is true of the southern parts of the country but to a lesser extent. In addition, the demand for gas varies during the day, with lesser amounts being required during evening hours than during the daylight period. This is because industry employs larger amounts, generally, during the daylight hours and because home consumption is generally greater due to cooking during the daylight and early evening hours than in the nighttime. v

Natural gas is distributed by means of pipelines which are usually unable to provide sufficient gas to supply the demand at peak periods.

In many areas of the country, it is necessary to store Vlarge quantities of natural gas for the purpose of peak shaving to provide the large supply of gas needed during intervals of high demand. Since the volume of vapor is immense it would be impracticable to store it in this form. Therefore, the gas is liquefied near the ultimate area of consumption and subsequently vaporized and distributed through local gas distribution systems for ultimate consumption.

The liquefied gas can be stored in large double-walled insulated tanks at a low temperature commensurate with a vapor pressure only slightly above atmospheric pressure. An internal vapor pressure of 15 p.s.i.a., with the storage temperature of the liquefied gas being about 258 F., is representative of the storage conditions.

Natural gas liquefaction is .presently accomplished by a process involving at least three refrigeration loops followed by expansion, or ashing, into the insulated storage tank. Each loop utilizes a refrigerant at a dilierent pressure-temperature level and, since methane, ethylene and propane are among those often employed, they introduce additional hazard and expense. These loops are difficult to synchronize and control and, as there exists a temperature difference between the condensing vapor and cooling liquid between each loop, power inefficiency is inherent in the system.

According to the present invention there are provided novel processes and aparatus for liquefying methane, such as inthe form of natural gas, which do not employ a "ice I determined vpressure followed by a substantial lowering of the temperature ofthe `incoming methane by-means of a closed-loop refrigeration system, vfurther lowering the ternperature of the socooled-incomingfmethane gas by heat exchange with liquid. methaneI and methane vapor removed Vfrom an insulated main storage tank lto liquefy the incoming methaneand then iiashing -the resulting liquefied methane into an insulated main storage tank to further lower the temperature of lthe liquid methane for storage thereinpat a temperature at which the gas givesl a vapor pressure close to but slightly above atmospheric pressure. Advisably, the incoming methane*` gas is maintained at the same substantiallyvincreased predetermined pressure during ythe cooling cycle and until immediately prior to liquefaction of the incoming methane.

ln the described process, cold methane for process recycling,is handled by a liquid pump rather than by a compressor which, necessarily designed and built for low temperature service, is expensive to build, maintain and operate. To generate sufficient pressure to move methane from the l-ow temperature storage tankto the much higher pressure of the distribution pipeline, a liquid pumpis also employed.

The process of this invention more specifically comprises receiving methane from a transmission pipeline,

vsuch as at about 300-p.s.i.a., compressing/the gas to a v of the compressedmethane, passing vthe so-cooled pressurized methane -through the tubeside of an evaporator vessel cooled with a liquid methane on theshell side removed from the storage tank, ythereby lowering thetemperature of the compressed methane gas still further while maintaining the pressure about that to which it was originally compressed,r educing the pressure-on the socooled methane gas to liquefy vthesame and then flashing the liquid methane intothe main storage tank to cool the methane to a storage temperature commensurate with a vapor pressure of about 15 p.s.i.a.

1 In a more vefficient embodiment of theiinvention, the methane gas cooled by the closed-loop refrigeration system employing achlorofluoroalkane gas as-the refrigerant is passed subsequently through thetube side of two consecutive evaporator vessels cooled'4 by the use of liquid methane removed from-the Imain storage tank and pumped to said vessels by useof inexpensive liquid-conveying pumps.

v The invention will now be-.describedwith more particularity in connection with,` the attached drawing which comprises a diagrammatic flow sheet of the process for liquefying natural gas.

With reference to the drawing,v natur al gas is received such as from a cross country transmissionline 9 at a ypressure of -about 3y00 p.s.i.a. -The -gas is conveyed to compressor Vl() whichincreasesits pressure, ,such as to usually notbe above 200() p.s.i.a. 1 Pressures lower than cascade refrigeration syste-m and which, in addition, ren v 800 p.s.i.a. lead to a less etiicientoperation.

After thel gas pressure hasbeenincreased-it isv conveyed by pipe 12 through a Watercooler 13.which lowers f the temperature of the pressurized gas according to the temperature-of the coolingwater used. In some areas of the country the cold watery available .could .permit cooling-the gas down to about 50 F. or chilled Water may be used.

The pressurized gas is then passed through heat exchanger 14. It is cooled in this heat exchanger by passing therethrough natural gas vapor at low temperatures. The vapor is obtained from the main storage tank 40 as will be more clearly seen hereinafter. By means of the heat exchanger 14 the pressurized natural gas is cooled considerably, such as down to about 45 F. It then is conveyed by pipe 15 into the -tube side of vessel 16. The gas is cooled in vessel 16 by means of a liquefied chlorofluoroalkane type refrigerant boiling therein.

The vessel 16 is part of a closed-loop refrigeration system, as shown in the drawing, which employs a chloroffuoroalkane as the refrigerant. In this closed-loop refrigeration system 17, compressors 18 and 19 compress the gas and convey it through a water cooled heat exchanger 20 and then to compressor 21 from which it goes to cooled condenser 22 and then is piped to pressure tank receiver 23. The liquefied gas is then conveyed from vessel 23, such as at a temperature of about 50 F., by pipe 24 through heat exchanger 2S where it is cooled. The liquefied gas then passes through pipe 26, through expansion valve 27 and then into vessel 16 at a pressure where it boils at a low temperature of about 102 F. and a pressure of 2.2 p.s.i.a. The vapor from vessel 16 is conveyed from the shell side thereof through pipe 28 and into heat exchanger 25 from which it emerges at a temperature of about 50 F. and then passes to compressor 18. The cycle is then repeated in this closed-loop refrigeration system.

The closed-loop refrigeration system 17 advisably employs any suitable commercial refrigerant which can lower the temperature of the methane adequately such as to about 75 to 125 F. It is particularly advisable, however, to employ as the refrigerant a fluorochlorinated alkane having one or two carbons such as dichlorodiuoromethane `(Freon 12), dichlorotetrafiuoroethane (Freon 114), trichloromonoiluoromethane (Freon 11), dichloromonofluoromethane (Freon 21), and monochlorotriuoromethane (Freon 13), or mixtures of such gases. The particular specific data of this application is based on the use of dichlorodifluoromethane as the refrigerant. Adjustments in temperatures and pressures will of course be required with the use of any other such refrigerant gas of the described type but any such Variations are easily calculated for and taken care of.

Regardless of the refrigerant used the purpose is to lower the temperature of the methane on the tube side of vessel 16 as far as practicable with efficiency.

Referring again to the natural gas, it is cooled in vessel 16 to a substantially lower temperature than at which it entered. Thus it can leave vessel 16 at a temperature of about 98 F. and be conveyed by pipe 29 to pipe 30 and from pipe 30 through pipe 31 into heat exchanger 32. 1n heat exchanger 32 the temperature of the natural gas is lowered such as to about 154 F. This lowering of the temperature is effected by means of natural gas being passed through the other side of the exchanger at a lower temperature. This natural gas is obtained indirectly from the main storage tank 40 as will be shown hereinafter.

Upon leaving heat exchanger 32 the cooled natural gas, still at the same pressure as created by compressor 10, viz. 1000 p.s.i.a., is conveyed at a temperature such as about 154 F. by means of pipe 33 to the tube side of vessel 34. Vessel 34 is cooled by means of liquid natural gas boiling therein on the shell side. The liquid natural gas used for this cooling operation is removed Vfrom the main storage tank 40 by pipe 35 and by means of an inexpensive liquid pump 36 is conveyed by pipe 37 to the vessel 34. The liquid methane in the main storage tank 40 is maintained at a temperature of 258 F. and at a pressure of about 15 p.s.i.a.

The liquid methane is advisably used in vessel 34 at a temperature which will permit it to vaporize lat a pressure such that it can be recirculated directly into a transmission or consumer pipeline. Where it is desired that the vapor removed from vessel 34 be sent directly to a consumer line operating at a low pressure, such as p.s.i.a., the gas may be removed directly at that pressure by employing a suitable boiling temperature in vessel 34. Thus, the temperature of the liquid methane in vessel 34 can be at 210 F. for the vapor to boil off at 90 p.s.i.a., which is a conventional pressure used in consumer lines delivering natural gas. The vapor leaving vessel 34 can be conveyed by pipe 38 through heat exchanger 32 and then by pipe '75 into heat exchanger 14 for cooling incoming natural gas. The outgoing gas, upon leaving heat exchanger 14, can be conveyed by conduit 39 at 90 p.s.i.a. directly to a consumer line 41 or it can be directed through line 42 to compressor 43 and the pressure thereof raised, such as to 300 p.s.i.a., for conveying by pipe 44 to pipe 9 for recirculation in the stream. Thus, the methane which is recirculated completes a closed-loop cooling system which utilizes liquefied natural gas from the main storage tank 46 for cooling such as by vessel 34.

The natural gas, as it leaves vessel 34, has been further substantially cooled, such as down to 206 F., while maintaining the same pressure, such as 1000 p.s.i.a., produced by means of compressor 10. It is therefore seen that no compressors of high compression ratio are needed in the very low temperature part of the liquefaction system.

The very cold gas is removed from vessel 34 by pipe 44 and is sent through pressure control valve 45 into liquefied gas receiver 46 which is maintained at about p.s.i.a. The liquid level in the vessel 46 is maintained at a predetermined position by means of control system 47 which is interconnected with control valve 48 in line 49. Vapor is removed from vessel 46 by line 4. This vapor is generally very high in nitrogen but, nevertheless, is suitable for operating gas engines for furnishing power to operate various pumps and compressors in the system. The liquefied natural gas or methane is then conveyed by pipe 50 into the main storage tank 40 where it is flashed to a temperature of about 258 F. for storage at approximately atmospheric pressure or slightly thereabove, such as at 15 p.s.i.a.

Vapors formed during the final flash, as Well as vapors formed due to heat-leak between the storage tank 40 and its surroundings, plus vapor displaced by the fill, are removed by line 51 and by means of a low compression ratio compressor 52, are increased to a pressure of about 25 p.s.i.a. The vapors are then conducted by pipe 53 :through the tube side of evaporator-condenser vessel 60 where they are liquefied and then conveyed by pipe 54 into a receiver 55. Pump 56 then conveys the liquefied gas from receiver 55 to pipe 57 and into the bottom of the main storage tank 40.

Vessel 60 is cooled by withdrawing liquefied natural gas from the main storage tank 40 by means of pipe 35 and then sending it to pump 58 by which the liquefied gas is pumped through pipe 59 into vessel 60. The liquefied natural gas in evaporator-condenser 60 is at about 254 F. and the vapor at 18 p.s.i.a. The gas leaves the shell side of vessel 60 and is conveyed by pipe 61 into heat exchanger 32 from which it leaves by pipe 62 which feeds to blower 63. Blower 63 sends the gas through heat exchanger 14 and by means of pipe 64 it is conveyed to compressors 65 and 66 which increase its pressure, such as to 90 p.s.i.a., for delivery directly to a consumer line.

Although so far in this discussion the invention has been described as a process which can employ only -a single evaporator-condenser vessel which uses as its coolant liquefied natural gas withdrawn from the main storage tank for cooling the partially cooled natural gas :after it leaves the evaporator-condenser vessel 16, it is of course clear that .a series of evaporator-condenser vessels which employ liquefied natural gas withdrawn from the main storage vessel as the coolant can be employed, subsequent to the closed-loop refrigeration system using a chlorofiuoroalkane gas as the refrigerant, for lowering the temperature of the natural gas sufficiently far so that it can be liquefied and then ashed into the main storage tank for storage at a temperature commensurate with a vapor pressure not significantly greater than atmospheric pressure.

With reference to the drawing, it will be seen that the 'process as illustrated therein utilizes a second evaporator .vessel 70 for lowering the temperature of the natural gas after it leaves the closed-loop refrigeration cycle. Thus, after the natural gas leaves vessel 16, it can be conveyed through pipe 29 into pipe 71 .and through the tube side of vessel 70. The gas enters vessel 70 at about 98 F. and is cooled therein to a temperature of about 130 F. The so-cool'ed natural gas is conveyed from vessel 70 by pipe 72 into previously described pipe 31 and would thereafter follow the previously described route for further cooling in vessel 34.

The coolant for evaporator-condenser 70 is liquefied natural gas removed from the main storage tank 40 and conveyed by pipe 35 to pump 80 from which it emerges to pipe 81 for conveyance to the shell side of vessel 70. The temperature of the liquefied natural gas in vessel 70 is raised to 160 F. which is commensurate with a pressure of 300 p.s.i.a. The evaporated gas leaves vessel 70 by means of pipe 82 and passes through heat exchanger 14 for cooling incoming natural gas in line V12. The gas from pipe 82 leaves heat exchanger 14 by pipe 83 and is then recirculated into the system by means of pipe 9. Since the gas leaves vessel 70 at about 300 p.s.i.a., there kis no need for increasing its pressure before it is recirculated into the system since the feedline 9 supplies the gas originally at such pressure. There is, therefore, no unnecessary work loss involved in changing the pressure of the gas as it is being recirculated.

To start up the system, it is not necessary that the main storage tank contain a supply yof liquefied natural gas. The closed-loop refrigeration system operating with a lchlorofluoroalkane refrigerant will cool the gas sufficiently in vessel 16 so that upon being ashed subsequently into the main storage tank 40 -it will lead to the production of some liquid methane although mostly vapor. This vapor can be ared off by pipe 90 until there is sufficient liquid natural gas available in the main storage tank for Arecycling in the system. Alternatively, the vapor from pipe 90 can be recirculatedback into the system. Since it is cool, however, various heat exchangers, not shown, could be yused to 4rernove refrigeration therefrom for cooling incoming natural gas.

Various changes and modifications of the invention can be made and, to the extent that such variations incorporate the spirit of this invention, they areintended to be included within the scope of the appended claims.

What is claimed is:

1. The method of liquefying methane which comprises subjecting incoming methane gas to a substantially increased predetermined pressure, substantially lowering the temperature of the incoming methane vby means of `a closed-loop refrigeration system, further lowering the temperature of the so-cooled incoming methane gas by means of liquid methane removed from an insulated main storage tank for liquid met-haue to further reduce the temperature of the incoming methane, expanding the gaseous methane to liquefy and then flashing `the resulting liquefied methane into the main .storage tank to further lower the tempe-rature of the liquid methane for storage at a temperature at which the gas gives a vapor pressure close to but slightly above atmospheric pressure.

2. The method of claim 1 in which the closed-loop refrigeration system uses a chlorouoroalkane gas as the refrigerant.

3. The method of claim 1 in which a liquid pump furnishes pressure at the low temperature end of the methane liquefaction cycle to withdraw liquid methane from the main storage tank for use in cooling the incoming methane gas vand to propel vapor formed in the `system.

4. The method of claim 1 in lwhich the increased predetermined pressure provides the only propelling means for moving the incoming methane through the system and to the main storage tank.

5. A method of liquefying natural gas and storing the same in an insulated storage tank at close to but slightly above atmospheric pressure, which comprises subjecting incoming natural gas to a substantially increased predetermined pressure, cooling the so pressurized natural gas by means of a closed-loop refrigeration system which employs a chlorofluoroalkane gas as the refrigerant, further cooling the incoming natural gas by passing 'it through the tube side of a first vessel cooled on the shell side by liquefied natural gas pumped from the insulated storage tank, removing the incoming natural gas from the tube side of the first vessel at a much lower temperature than at which it enters, passing 'the lincoming natural gas through the tube side of a second vessel vcooled on the shell side by liquefied natural gas pumped from the insuilated storage tank to further lower the temperature of the incoming natural gas, said second vessel being at a much lower temperature than said first vessel, expanding the natural gas to liquefy and ashing the expanded liquefied natural gas into said storage tank for storage at a temperature at which the gas gives a vapor pressure close to but slightly above atmospheric pressure.

6. The method of cliam 6 in which the substantially increased predetermined pressure is about 800 to 2000 p.s.1.a.

7. A method of liquefying a compressed natural gas stream including a recycled vaporized methane product derived therefrom and storing liquefiedmethane product in an insulated storage tank at a low temperature wherein the pressure is only slightly above atmospheric, comprising the steps of:

(l) compressing a natural gas stream and the said recycled vapors to a substantially increased predetermined pressure of about 800 to 2,000 p.s.i.a. to form a main inlet natural gas stream,

(2) passing the inlet natural gas stream in heat exchange with said vaporized methane product of the process to substantially precool the inlet natural gas stream,

(3) further cooling the inlet natural gas stream with a closed-loop Freon external refrigeration cycle,

(4) further reducing the temperature of the inlet natural gas stream by stepwise heat exchange in series of a first and second evaporator-cooler cooled with liquid methane product from the process,

(5) expanding the so-cooled inlet natural gas stream to form a liquefied methane product and residual gases,

(6) further flashing the liquefied product into the said insulated storage tank ata low temperature and a pressure only slightly above atmospheric, forming a low temperature liquid methane-product and a second residual gas,

(7) removing liquefied methane product from the insulated storage tank and pumping two separate streams to the first and second evaporator-coolers to condense the inlet natural gas stream and thus to vaporize the removed methane liquid product,

(8) passing the sovaporized methane liquid product in heat exchange with the inlet natural gas stream of step (2) to precool said stream,

(9) recycling the vaporized methane liquid product from the first evaporator-cooler of step (7) back to the suction of the compressor in step (1) after the vaporized methane liquid has been heat exchanged in step (8),

(10) passing the vaporized methane liquid product from the second evaporator of step (7) to a pipe line for use after the vaporized methane liquid has been heat exchanged in step (8),

(11) removing the second residual gas of step (6) from storage, compressing said gas, liquefying said gas in a third evaporator-cooler by means of low temperature liquid methane product removed from the insulated storage tank, returning the now liquetied second residual gas to the storage tank and forming a third vaporized liquid methane product stream, and

(12) passing the third vaporized liquid methane product stream in heat exchange with the inlet gas of step (2) before passing it to pipe line use of step (10).

8. A method of liquefying a compressed natural gas stream and storing liquefied methane product in an insulted storage tank at a low temperature wherein the pressure is only slightly above atmospheric, comprising the steps of:

(1) compressing a natural gas stream to a substantially increased predetermined pressure to form a main inlet natural gas stream,

(2) passing the inlet natural gas stream in heat exchange with said vaporized methane product of the process to substantially precool the inlet natural gas stream,

(3) further cooling the inlet natural gas stream with a closed-loop external refrigeration cycle,

(4) further reducing the temperature of the inlet natural gas stream by stepwise the-at exchange in series of a first and second evaporator-cooler with liquid methane product from the process,

(5) expanding the so-cooled inlet natural gas stream to form a liquefied methane product and residual gases,

(6) further flashing the liquefied product into the said insulated storage tank at a low temperature and a pressure only slightly above atmospheric, forming a low temperature liquid methane product and a second residual gas,

(7) removing liquefied methane product from the in sulated storage tank and pumping two separate streams of it to the rst and second evaporatorcoolers to condense the inlet natural gas stream and thus to vaporize the removed methane liquid product,

(8) passing the so-vaporized methane liquid product in heat exchange with the inlet natural gas stream of step (2) to precool said stream,

(9) passing the vaporized methane liquid product from the first and second evaporators of step (7) to either a pipe line or back to the suction of the compressor in step (1),

(10) removing the second residual gas of step (7) from the storage tank, comprising said gas, liquefying said gas in a third evaporator-cooler by means of low temperature liquid methane product removed from the insulated storage tank, returning the now liquefied second residual gas to the storage tank and forming a third vaporized liquid methane product stream, and

(11) passing the third vaporized liquid methane product stream in heat exchange with the inlet gas of step (2) before passing it to a pipe line or compressor as in step (9).

9. A method of liquefying a compressed natural gas stream and storing liquefied methane pnoduct in an insulated storage tank at a low temperature wherein the pressure is only slightly above atmospheric, comprising the steps of (1) compressing a natural gas stream to a substantially increased predetermined pressure to form a main inlet natural gas stream,

(2) passing the inlet natural gas stream in heat exchange with said vaporized methane product of the process to substantially precool the inlet natural gas stream,

(3) further cooling the inlet natural gas stream with a closed-loop external refrigeration cycle,

(4) further reducing the temperature of the inlet natural gas -stream by stepwise heat exchange in series of a iirst and second evaporator-cooler with liquid methane product from the process,

(5) expanding the so-cooled natural gas stream to form a liquefied methane product and residual gases,

(6) further flashing the liquefied product into the said insulated storage tank at a low temperature and a` pressure only slightly above atmospheric, forming a low temperature liquid methane product and a second residual gas,

(7) removing liquefied methane product from the insulated storage tank and pumping two separate streams of it to the rst and second evaporator-coolers to condense the inlet natural gas stream and thus to vaporize the removed methane liquid product,

(8) passing the so-vaporized methane liquid product in heat exchange with the inlet natural gas stream of step (2) to precool said stream, and

(9) passing the vaporized methane liquid product from the iirst and second evaporators of step (7 to a pipe line or back to the suction of the compressor in step (l).

References Cited by the Examiner UNITED STATES PATENTS Swenson 62-40 X Cicalese 62-20 X De Lury 62-40 X NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, J. T. JOHNSON, Assistant Examiners.

Knapp 62--23 Xl 

1. THE METHOD OF LIQUEFYING METHANE WHICH COMPRISES SUBJECTING INCOMING METHANE GAS TO A SUBSTANTIALLY INCREASED PREDETERMINED PRESSURED, SUBSTANTIALLY LOWERING THE TEMPERATURE OF THE INCOMING METHANE BY MEANS OF A CLOSED-LOOP REFRIGERATION SYSTEM, FURTHER LOWERING THE TEMPERATURES OF THE SO-COOLED INCOMING METHANE GAS BY MEANS OF LIQUID METHANE REMOVED FROM AN INSULATED MAIN STORAGE 